This study tests the fidelity of shallow-water gastropod skeletons as multiproxy archives of paleoenvironmental change by comparing isotopic and trace-metal analyses of specimens of Conus ermineus. Four adult specimens were collected live from Stetson Bank in the northwestern Gulf of Mexico during the summer of 2003. Shells were sampled along axes of growth to produce time-series profiles spanning up to 8 years. δ¹⁸O and Sr/Ca profiles show seasonal cyclicity modified by fast summer and slow winter shell growth. The profiles were combined to estimate paleosalinity. This yields variable results that overestimate salinity range; nevertheless, annual salinity minima and maxima are still evident. The overestimates are attributed to interspecimen Sr/Ca variability and error in the δ¹⁸O_sw-salinity regression. Profiles of δ¹³C show seasonal variation superimposed on a decreasing ontogenetic trend, the latter ascribed to decreasing metabolic efficiency also reflected by an ontogenetic increase in Sr/Ca. Seasonal δ¹³C variation reflects changes in the δ¹³C of dissolved inorganic carbon (δ¹³C_DIC). Salinity and δ¹³C_DIC at Stetson Bank strongly correlate (R² = 0.80, p < 0.0001), and shell δ¹³C minima coincide with local salinity minima following times of peak river discharge. These δ¹³C minima terminate during annual shelf current reversals. Low-salinity waters directly account for less than half the variability in shell δ¹³C but enhance summer stratification and trap respired CO₂ from sediment pore waters. Specimens from this study show mean δ¹³C values 1‰ lower than C. ermineus collected from Stetson Bank in 1971, reflecting the decrease in δ¹³C_DIC from anthropogenic CO₂.